The present invention relates to a color cathode ray tube of the type which is equipped with an in-line type electron gun constructed to emit three electron beams horizontally in one row toward a phosphor screen.
In a color cathode ray tube, a vacuum vessel is constructed of a panel portion providing a display screen, a neck portion having an electron gun assembly disposed therein, and a funnel portion joining the panel portion and the neck portion.
In an electron gun assembly arranged in the neck portion, three electron guns are arrayed in-line at a spacing s for emitting three electron beams for individually irradiating red (R), green (G) and blue (B) color phosphors of a phosphor screen formed on the inner face of the panel portion. On the phosphor screen, there are arranged individual phosphors which are adjacent to each other for the red (R), green (G) and blue (B) colors to form one color pixel.
The three electron beams, as emitted from the individual electron guns, are able to irradiate the individual phosphors corresponding to each color pixel by the actions of a deflection yoke (hereinafter to be referred to as the xe2x80x9cDYxe2x80x9d) which is mounted generally around the boundary between the neck portion and the funnel portion. In order to adjust the trajectories of the electron beams so that the individual electron beams, as deflected by the DY, may irradiate predetermined phosphors accurately, an adjustment magnet arrangement is mounted around the neck portion. This adjustment magnet arrangement is constructed, for example, of 2-pole and 4-pole magnets disposed on the side of the DY, and a magnet assembly composed of 2-pole, 4-pole and 6-pole magnets disposed on the side of the electron gun assembly.
As an example of a color cathode tube having the aforementioned construction, there has been proposed a color cathode ray tube which has an enhanced deflection sensitivity obtained by reducing the external diameter of the neck portion, as disclosed in Japanese Patent Laid-Open No. 7-141999 (Japanese Patent Application No. 5-286772).
However, when a color cathode ray tube is constructed in such a way as to reduce the external diameter of the neck portion to 24.3 mm (from a conventional diameter of 29.5 mm) and, accordingly, to reduce the s-size (electron beam spacing at the main lens of the electron gun assembly, hereinafter to be referred to as the xe2x80x9cs-sizexe2x80x9d) of the electron guns to 4.75 mm (from the conventional size of 5.5 mm), the relative tolerances normalized by either the s-size or the size of the external diameter of the neck portion are increased, if the electron gun and sealing tolerances have been set likewise for the large external diameter neck portion. Then, it can be operated without adjusting the shifts of the electron beams to large values.
When the shift adjustment by the 2-pole magnet of the adjustment magnet arrangement thus increases, there arises a difference among the amounts of shift of the individual electron beams of the red (R), green (G) and blue (B) colors. Thus, the 6-pole and 4-pole magnets of the magnet assembly have to act upon the individual electron beams to adjust the aforementioned difference in the amounts of shift. As a result, the electron beams are shifted at first by the 6-pole and 4 pole magnets of the magnet assembly so that their center trajectories fail to follow the axis of the main lens of the electron gun.
When the center trajectories of the electron beams follow paths shifted upward of the lens center, for example, the upper portions of the electron beams come closer to the electrode than the lower portions so that the upper portions of the beams are more focused than the lower portions. As a result, there appears a phenomenon in which the focuses of the beams are offset at the upper and lower portions. Even if the focus of the main lens is adjusted by the electrode voltage, therefore, the upper and lower portions of the electron beams cannot be simultaneously focused to an optimum degree. As a result, the outer peripheral portions (or a so-called xe2x80x9chaloxe2x80x9d) of the electron beams are offset in shape. When this halo exceeds an allowable range, the focusing characteristics are deteriorated, thereby to degrade the display image.
When the 2-pole magnet of the magnet assembly is activated, there will also arise a difference in the amounts of shift of the individual electron beams of the red (R), green (G) and blue (B) colors. If the 2-pole magnet is placed very much closer to the 4-pole and 6-pole magnets, however, this shift difference is compensated by the adjoining 4-pole and 6-pole magnets, so that the difference in the individual amount of shift can be adjusted to reduce the misalignment of the electron beams in the main lens.
In other words, the aforementioned phenomenon, i.e. the halo offset, becomes more noticeable for the case in which the 2-pole magnet for color purity adjustment is located at a back stage, i.e., away from the 4-pole and 6-pole magnets, which are normally located at a front stage relative to the main lens.
An object of the invention is to provide a color cathode ray tube which can reduce the focusing defect of the offset halo and can improve the reliability, even if the 2-pole magnet is located away from the 4-pole and 6-pole magnets.
According to a feature of the invention, there is provided a color cathode ray tube comprising: a vacuum vessel including a panel portion having a phosphor screen on its inner face, a neck portion and a funnel portion joining the neck portion and the panel portion; an electron gun assembly including an electrostatic main lens disposed in the neck portion; a deflection yoke arranged around the neck side of the funnel portion for deflecting the three in-line arranged electron beams which are emitted from the electron gun assembly to the phosphor screen; and a 2-pole magnet arranged around the neck portion for adjusting the trajectories of the electron beams. The 2-pole magnet is arranged to have its center closer to the phosphor screen than the center of the electrostatic lens of the electron gun assembly. The value, as calculated by dividing the value of the radial component amplitude of the magnetic field distribution of the 2-pole magnet on the circumference of a circle having a radius of the e-size, by the value of the circumferential component amplitude, is 0.86 to 1.38, are preferably 0.955 to 1.275. The color cathode ray tube thus constructed according to the invention can reduce the focusing defect drastically, as might otherwise be caused by the halo effect.